A transport provider that wishes to offer high capacity facilities to customers can implement the offer by simply providing so-called “dark fiber,” allowing the customers to place whatever signals they wish on the fiber.
One value proposition is for the provider to offer a fiber and a “service,” whereby a channel is provided for transmission of information, with a guarantee that the transmitted information will arrive at its destination with an agreed-to quality of service. To provide the agreed-to quality of service, the provider sends the information over the fiber in a particular protocol that is chosen by the provider, monitors the quality of the service, and performs appropriate maintenance activities, as needed. That means that the provider carries on the fiber various signals that do not belong to the customer for the purpose of monitoring the quality of service. Dark fiber clearly cannot meet this value proposition.
An advanced value proposition is for the provider to offer a fiber, and to also offer a plurality of channels, concurrently, using a particular protocol, with the channels adapted to carry client signals. SONET is an example of such a value proposition. SONET encapsulates a client-provided signal into successive Synchronous Payload Envelope (SPE) blocks of data, injects these blocks into successive SONET frames, modulates numerous SONET frames onto different wavelengths, and places them onto a fiber. The reverse process takes place when data needs to be extracted.
One aspect of SONET is that it offers clients a variety of bandwidths. The lowest SONET bandwidth (OC-1) is capable of carrying a DS3 signal, having a 44,736 Mb/s rate, and the SONET standard contemplates higher bandwidths in multiples of OC-1. However, commercial equipment that carries SONET signals over fiber handles only OC-3, OC-12, OC-48, and OC-192 signals. Intermediate rates are generally multiplexed into one of these four signal rates.
Another aspect of SONET is that it can be add/drop multiplexed, meaning that a given channel can be extracted from, or added to, the information signal that is contained in a given wavelength without having to extract all of the other channels that are contained in the information signal, or to reconstitute the information signal.
Still another aspect of SONET is that it carries it's own maintenance information, permitting the provider to offer a guaranteed level of service quality without having to delve into the client's signal per se.
What would be desirable that SONET cannot provide is the ability to transmit client signals that themselves are SONET frames, transparently, and in a bandwidth efficient manner while maintaining the timing integrity of the client SONET signals themselves. By “transparently” what is meant is that the offered client's signal (e.g., an OC-3 SONET signal) can be communicated through the network, from an ingress node to an egress node, in a manner that allows the client's signal to be multiplexed onto a fiber with one or more other signals, where the other signals possibly have different bandwidths, or different protocols, and where the other signals may be time-division-multiplexed onto the same wavelength, or onto different wavelengths, the clients signal can have any desired protocol (i.e., including SONET), the client's signal can be add/drop multiplexed at any point in the network without requiring add/drop operations on other signals and, correspondingly, add/drop operations need not be undertaken relative to the client's signals when add/drop multiplexing is performed on some other signal on the fiber, and the provider is able to ascertain quality of service provided to the client without having to look into the client's signal per se.
As indicated above, SONET fulfills the above transparency requirements, except that it does not allow the client to send a signal that itself follows the SONET protocol while maintaining the timing integrity of the SONET client signal. Clearly, for example, one cannot send an OC-3 SONET client data frame as a unit over an OC-3 SONET frame, because the payload bandwidth of the provider's OC-3 frames is simply not large enough to carry both the payload and the overhead of the client's signal. One possibility that has been studied by Lucent Technologies is to stuff an OC-3 frame into an OC-4 signal. After extensive efforts it was concluded that this proposal was not able to meet the SONET timing standards for the client SONET signal. This is clearly evident in FIG. 5-18(a) of T1X1.3/2002-036 contribution to the T1 Standard Project-T1X1.3. This contribution, titled “Jitter and Wander Accumulation for SONET/SDH over SONET/SDJ (SoS) Transport” by Geoffrey Garner, dated Sep. 30, 2002, which is hereby incorporated by reference. Note that all simulation depicted in the aforementioned FIG. 5-18(a) are above the OCN Reference Mask; where the need is to be below this mask.
Separately, the Digital Wrapper standard exists (G.709) that contemplates signals flowing in frames having one of three line rates. The lowest rate (OTU1) carries 20,420 frames/s, and each frame consists of 16,320 bytes that structurally can be viewed as 4 rows and 4080 columns. Sixteen columns are devoted to overhead, 3808 columns are devoted to client payload, and 256 columns are devoted to forward error correction, which results in a payload rate of approximately 2.666 Gb/s. The OTU1 rate can be used to communicate a 2.48832 Gb/s OC-48 SONET signal, as the payload area was sized for that capacity. Equipment exists to terminate a number of SONET signals and, after removing their payload information (SPE), multiplex the individual payloads to form an OC-48 signal, to encapsulate it in an OTU1 digital wrapper, and to modulate the resulting signal onto a chosen wavelength. To date, however, no design exists for channelizing the Digital Wrapper for the many lower rate data services that a telecommunications carrier is called upon to transport, such as the above-mentioned OC-3 signal, i.e., a design that allows one to carry sub-multiples of the OTU1 signal (also termed sub-wavelength channels) using the Digital Wrapper standard.